A modular design of metal catalysts for the transfer hydrogenation of aromatic ketones

The ability of transition metal catalysts to add or remove hydrogen from organic substrates by transfer hydrogenation is a valuable synthetic tool. Towards a series of novel metal complexes with a P-NH ligand, [Ph2PNHCH2C4H3O] derived from furfurylamine were synthesized. Reaction of [Ph2PNHCH2C4H3O] 1 with [Ru(6-p-cymene)(mu-Cl)Cl]2, [Ru(6-benzene)(mu-Cl)Cl]2, [Rh(mu-Cl)(cod)]2 and [Ir(5-C5Me5)(mu-Cl)Cl]2 gave a range of new monodentate complexes [Ru(Ph2PNHCH2C4H3O)(6-p-cymene)Cl2] 2, [Ru(Ph2PNHCH2C4H3O)(6-benzene)Cl2] 3, [Rh(Ph2PNHCH2-C4H3O)(cod)Cl] 4, and [Ir(Ph2PNHCH2-C4H30)(5-C5Me5)Cl2] 5, respectively. All new complexes were fully characterized by analytical and spectroscopic methods. 31P-{1H} NMR, distortionless enhancement by polarization transfer (DEPT) or 1H-13C heteronuclear correlation (HETCOR) experiments were used to confirm the spectral assignments. Following activation by KOH, compounds 25 catalyzed the transfer hydrogenation of acetophenone derivatives to 1-phenylethanol derivatives in the presence of iso-PrOH as the hydrogen source. Notably [Ru(Ph2PNHCH2-C4H3O)(6-benzene)Cl2] 3 acts as an excellent catalyst, giving the corresponding alcohols in 9899% yield in 20min at 82 degrees C (time of flight=297h-1) for the transfer hydrogenation reaction in comparison to analogous rhodium or iridium complexes. Copyright (C) 2012 John Wiley & Sons, Ltd.